Direct bonding is a commonly used bonding method in the manufacturing of integrated circuits. In the direct bonding, two metal bumps are bonded together without solder disposed in between. For example, the direct bonding may be a copper-to-copper bonding or a gold-to-gold bonding. The methods for performing the direct bonding include Thermal Compression Bonding (TCB, sometimes known as thermal compressive bonding). In a direct bonding process, the metal bumps of a device die is aligned to, and placed against, the metal bumps of a package substrate. A pressure is applied to press the device die and the package substrate against each other. During the bonding process, the device die and the package substrate are also heated. With the pressure and the elevated temperature, the surface portions of the metal bumps of the device die and the package substrate inter-diffuse, so that bonds are formed. Solder layer with thickness less than 3 μm may be added to each side of the metal bumps of the device die and the package substrate as the top portions of the respective metal bumps. In the direct bonding, the solder layers are in contact with each other, and are bonded with the underlying non-flowable portion of the metal bumps.
To allow the inter-diffusion to occur, the direct bonding is usually a lengthy process, sometimes taking hours or days to finish. The throughput of the direct bonding is thus low.